Step 1: Understanding the context.
In the given extract, the poet expresses deep emotional pain. The poet says, "Sometimes I feel myself I can hardly bear," which indicates an overwhelming sense of despair or emotional struggle.
Step 2: Identifying the subject of pain.
The poet can hardly bear the feeling of "childish longing in vain." This longing refers to the desire or wish that remains unfulfilled, causing the poet great emotional distress.
Step 3: Conclusion.
Thus, the poet can hardly bear the feeling of unfulfilled desires and the pain that comes with longing for something unattainable.
Final Answer: \[ \boxed{\text{The poet can hardly bear the feeling of childish longing that remains unfulfilled.}} \]